"Copying of any video game for any Nintendo system is illegal and is strictly prohibited by domestic and international intellectual property laws. "Back-up" or "archival" copies are not authorized and are not necessary to protect your software. Violators will be prosecuted" -F-Zero GX Manual (Though likely in all other Nintendo manuals)
But, as was previously said, it's doubtful officers will come knocking over a video game on an outdated console.
Federal law Trumps this. In some countries, yes its definitely illegal. But in the US as stated previously with links directly to copyright.gov its legal. You just forfeit any sort of tech support/warranty with Nintendo if you do it. Over course, nobody has an official warranty on their GCN stufg these days anyways, so have at it